Update UPGRADE.txt for 13 branch

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@420497 f38db490-d61c-443f-a65b-d21fe96a405b
Remove 12 merge properties

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@420498 f38db490-d61c-443f-a65b-d21fe96a405b
Initialize svnmerge from branches/13

git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@420499 f38db490-d61c-443f-a65b-d21fe96a405b